1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, a process cartridge and an electrophotographic apparatus, and more particularly to an electrophotographic photosensitive member, a process cartridge and an electrophotographic apparatus which are suited for short-wavelength semiconductor lasers capable of making images have higher resolution.
2. Related Background Art
In electrophotographic apparatus making use of lasers as light sources as typified by laser printers, semiconductor lasers having oscillation wavelength around 800 nm or around 680 nm are prevailingly used. In recent years, various approaches to higher resolution are made with an increase in demand for reproducing images having a higher image quality. Wavelengths of lasers also deeply concern the higher resolution. As disclosed in Japanese Patent Application Laid-Open No. 9-240051, the shorter oscillation wavelength a laser has, the smaller spot diameter the laser can have. This enables formation of latent images having a high resolution.
Some methods are available for making laser oscillation wavelength shorter.
One of the methods is a method in which a non-linear optical material is utilized so that the wavelength of laser light is shortened to half by using secondary higher harmonic generation (SHG) (e.g., Japanese Patent Applications Laid-Open No. 9-275242, No. 9-189930 and No. 5-313033). This system can achieve a long life and a large output, since it can use GaAs semiconductor lasers or YAG lasers as primary light sources, which have already established their technique and can achieve a high output.
Another is a method in which a wide-gap semiconductor is used, and can make apparatus smaller in size than devices utilizing the SHG. ZnSe semiconductor lasers (e.g., Japanese Patent Applications Laid-Open No. 7-321409 and No. 6-334272) and GaN semiconductor lasers (e.g., Japanese Patent Applications Laid-Open No. 8-088441 and No. 7-335975) have long been studied in great deal because of their high emission efficiency.
It, however, has been difficult for these semiconductor lasers to be optimized in their device structure, crystal growth conditions and electrodes, and, because of defects in crystals, has been difficult to make long-time oscillation at room temperature, which is essential for putting them into practical use.
However, with progress of technological innovations on substrates and so forth, Nichia Kagaku Kogyo K.K. reported, in October, 1997, GaN semiconductor laser's continuous oscillation for 1,150 hours (condition: 50.degree. C.), and materialization for its practical use stands close at hand.
Japanese Patent Application Laid-Open No. 9-240051 discloses as a photosensitive member suited for 400 to 500 nm lasers a multi-layer photosensitive member in which a single layer or charge generation layer making use of .alpha.-type titanyl phthalocyanine is formed as the outermost layer. Studies made by the present inventors, however, have revealed that the use of such a material brings about such a problem that, because of a poor sensitivity and a very great memory especially for light of about 400 nm, photosensitive members may undergo great potential variations when used repeatedly.